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Host-to-Host Layer

This layer shields the upper layers from the process of sending data. Also provides an end-to-end connection between two devices during communication by performing sequencing, acknowledgments, checksums, and flow control. Applications using services at this layer can use two different protocols: TCP and UDP.

TCP (Transmission Control Protocol)

TCP provides a connection-oriented, reliable services to the applications that use its services.
Main Functions of TCP

Segments application layer data stream--

TCP accepts data from applications and segments it into a desirable size for transmission between itself and the remote devices. The segment size is determined while TCP is negotiating the connection between the two devices. Either device can dictate the segment size.

Provides acknowledgment times--

TCP maintains timers to identify when packets have taken too long to get to their destination. When an acknowledgment is not received for a packet and the timer expires, TCP will resend the packet to the destination.

Enables sequence number checking--

TCP/IP uses sequence numbers to ensure that all packets sent by an application on one device are read in the correct order by an application on another device. The packets might not be received at the transport layer in the correct order, but TCP sequences them in their original order before passing them to the application layer.

Provides buffer management--

Any time two devices are communicating, the possibility exists that one device can send data faster than the other can accept it. If this happens, the receiving device puts the extra packets into a buffer to be read at the first chance it gets. When this data overflow persists, however, the buffer is eventually filled and packets begin to drop. TCP performs some preventive maintenance called flow control to avoid the problem.

Initiates connections with 3-way handshake--

TCP uses the concept of the three-way handshake to initiate a connection between two devices. A TCP connection begins with a device sending a request to synchronize sequence numbers (a SYN packet) and initiate a connection. The other device receives the message and responds with a SYN message and the sequence number increased by one. The first device responds by sending an acknowledgment message (an ACK) to the second device, indicating that the device received the sequence number it expected.

Performs error and duplication checking--

TCP uses a checksum to identify packets that have changed during transport. If a device receives a packet with a bad checksum, it drops the packet and does not send an acknowledgment for the packet. So the sending device will resend the packet. Any time TCP receives a duplicate packet it will drop the duplicate.

Performs acknowledgment windowing--

Any time a TCP device sends data to another device, it must wait for the acknowledgment that this data was received, To increase the bandwidth utilization, TCP can change the window size. Whatever the window size is negotiated to be, acknowledgments will only be sent after that many packets have been received at the receiving device. TCP sets the window size dynamically during a connection, allowing either device involved in the communication to slow down the sending data rate based on the other devices capacity. This process is known as sliding window because of TCP's ability to change the window size dynamically.

Before data is sent, the transmitting host contacts the receiving host to set up a connection known as a virtual circuit. This makes TCP connection-oriented. During the handshake the two hosts agree upon the amount of information to be sent before an acknowledgment is needed (Windowing). TCP takes the large blocks of data from the upper layers and breaks them up into segments that it numbers and sequences. TCP will the pass the segments to the network layer, which will route them through the Internetwork. The receiving TCP can put the segments back into order. After packets are sent, TCP waits for an acknowledgment from the receiving end of the virtual circuit. If no acknowledgment is received then the sending host will retransmit the segment.

UDP (User Datagram Protocol)

UDP transports information that doesn't require reliable delivery; therefore it can have less overhead than TCP as no sequencing or acknowledgments are used. NFS and SNMP use UDP for their sessions, the applications have their own methods to ensure reliability. UDP receives blocks of information from the upper layers, which it breaks into segments. It gives each segment a number, sends it, and then forgets about it. No acknowledgments, no virtual circuits, connectionless protocol.

Differences between TCP and UDP

TCP and UDP use port numbers to communicate with the upper layers. Port numbers keep track of different sessions across the network. The source port will be above 1024 (unprivileged). 1023 and below (privileged) are known as well known ports and are assigned to common protocols. TCP and upper layer don't use hardware (MAC) and logical (IP) addresses to see the host's address; instead they use port numbers.